J277 COMPUTER SCIENCE

Unit 1

CPU

CPU

"Responsible for carrying out the Fetch-Decode-Execute cycle and consisting of the registers, ALU and control unit."

Fetch-decode-execute cycle

The process carried out by the CPU where instructions and/or data are FETCHED from memory, Processed and Executed repeatedly until the program is completed.

ALU

"Performs calculations (e.g., x = 2 + 3) and logical comparisons (e.g., IF x > 3) in the CPU."

CU

"Decodes instructions and controls how data moves around the CPU."

Cache

Very fast memory that is usually on the processor. It is used to store frequently used data and/or instuctions.

Register

"Tiny areas of extremely fast memory located in the CPU, normally designed for a specific purpose to store data or information temporarily - e.g. MAR, MDR, etc."

Von Neumann architecture

Also known as the Stored Program Computer, this is the most common aarchitecture for modern CPUs

MAR

Stores the address of the next address in memory that will be accessed or fetched from. The address stored in the MAR could change multiple times in one FDE cycle.

N.B. This is a similar definition to the PC but it is important to be able to distinguish between them. "

MDR

"Holds data fetched from or to be written to memory."

Program counter

"Holds the address of the next instruction to be executed.."

Accumulator

"Holds the result of calculations."

Embedded system

A dedicated computer system that is built within a larger, mechanical or electronic device.

Primary storage

"Comprised of random-access memory (RAM) and read-only memory (ROM). It holds data and instructions that the CPU is using"

RAM

Volatile (data is lost when the computer is powered off). Read-and-write.

RAM

Purpose: Temporary storage of currently executing instructions and data - e.g., applications and the operating system.""

ROM

"Non-volatile (data is retained when the computer is powered off). Read-only. Purpose: Stores startup instructions, otherwise known as the bootstrap."

Virtual memory

"Using part of the hard disk as if it were random-access memory. Allows more applications to be open than RAM can hold."

Secondary storage

"Permanent storage of instructions and data not currently in use by the processor. Stores the operating system, applications and data. Read-and-write and non-volatile."

Optical storage

"CD-R, CD-RW, DVD-R, DVD-RW. Use: Music, films and archive files. Low capacity. Slow access speed. High portability. Prone to scratches. Low cost."

Magnetic storage

"Operating system and applications. High capacity. Medium data access speed. Low portability (except for portable drives). Reliable but not durable. Medium cost."

Solid-state storage

"Memory cards and solid-state hard drives (SSD). Use: Digital cameras and smartphones. Medium capacity. High portability. Reliable and durable. No moving parts. Fast data access speed. High cost."

Storage capacity

"The amount of data a storage device can store."

Storage speed

"The read/write access speed of a storage device."

Storage portability

"How easy it is to transport a storage device - e.g., solid-state and optical storage are highly portable, whereas magnetic storage is designed to stay in place."

Storage durability

"How resistant a storage device is to damage and wear. Devices with low durability are likely to fail earlier."

Storage reliability

"A relative measure of confidence that a storage device will function correctly and allow you to write, read, delete and modify data."

Overflow

"The generation of a number that is too large to be represented by the device intended to store it."

Hexadecimal

A number system that is used as it is easier to read and faster to record than binary for humans. Example of its use is for the recording of colour codes.

Binary shifts

"Allows you to easily multiply or divide a base-2 binary number. A left shift multiplies the number while a right shift divides.

Character set

"A set of symbols represented by a computer. These symbols, called characters, can include letters, digits, spaces, punctuation marks and control characters."

ASCII

"An early character that proved to be ideal for computer systems. 128 characters. "

Unicode

"Standard character set that replaces the use of multiple different character sets. Incorporates characters from almost all global languages. A 16-bit extension of ASCII."

Pixels

"The smallest unit of a digital image or graphic that can be displayed on a digital device. A pixel is represented by a dot or square on a computer display."

Metadata

"A collection of data that describes and provides information about other data." e.g. Date image taken, artist who recorded the song, resolution of image, length of song...

Colour depth

Used to record the number of colours that can be represented in a single pixel.

Resolution

"The number of pixels (individual points of colour) in a display"

Sample rate

""The number of samples taken per second, measured in hertz (Hz)."

Sample Rate

5Hz would mean 5 samples per second, 50Hz would mean 50 per second. "

Sample duration

"How many seconds of audio a sound file contains."

Sample bit depth

"The number of bits available to store each sample (e.g., 16-bit)."

Compression

"The process of reducing the size of a file."

Lossy compression

A method of compression where some data is actually removed from the original file. This can significantly reduce file size but also permanently removes data so the original file can not be restored.

Lossless compression

compresses the file without removing any data so the original file can be restored perfectly.

LAN

Local Area Network: "Small geographic area. All hardware is owned by the organisation using it. These can be both wired or wireless."

WAN

Wide Area Network: "Large geographic area. Not all hardware is owend by the organisations e.g. telephone lines hired to connect them."

Client-server network

"A network made up of client machines and one or more servers. A server controls access and security to one shared file store. A server manages access to the internet, shared printers and email services, as well as running data backups."

Peer-to-peer network

"All computers are equal and serve their own files to each other. Each computer is responsible for its own security and backups."

Wireless access point

"Hardware that allows a Wi-Fi-enabled device to connect to a network."

Router

""A router sends data packets between networks.

It is needed to connect a local area network to a wide area network.

It uses the IP address on a device to route traffic to other routers.""

Switch

""A switch sends data between computers on a local area network. It uses the NIC address on a device to route traffic.""

NIC

Network Interface Card/Controller: "Hardware that connects a computer to a network."

Transmission media

"Physical media that can be used to transmit data - e.g., twisted copper cable, fibre optic, etc."

The internet

"A worldwide collection of interconnected computer networks. An example of a WAN - the largest in existence."

DNS

Domain Name System: "The internet equivalent of the phone book. Maintains a directory of domain names and translates them to Internet Protocol (IP) addresses - this is necessary because, although domain names are easy to remember, computers access websites using IP addresses."

Hosting

"Websites stored on dedicated servers. Used for websites that need to be available 24/7, be accessed by thousands of users at a time, be well-protected from hackers and have an IP address that doesn't change."

The cloud

"Remote servers that store data to be accessed over the internet. Access anytime, anywhere from any device. Automatic backups. Collaborate on files easily."

Web server

"A program that uses HTTP (Hypertext Transfer Protocol) to deliver web pages to users. Page requests are forwarded by a computer's HTTP client. Dedicated computers and appliances may also be referred to as web servers."

Client

"A device that requests and/or uses services from a remote/connected server."

Network topology

"The physical or logical arrangement of connected devices on a network - e.g., computers, switches, routers, printers, servers, etc."

Star topology

""Computers connected to a central switch. If one computer fails, no others are affected. If the switch fails, all connections are affected.""

Mesh topology

""Switches/routers connected so there is more than one route to the destination - e.g., the internet. More resilient to faults but more cable is required.""

Wired connection

"Any computer network that predominantly connects hardware via physical cables - e.g., copper, fibre optic, etc."

Ethernet

"A standard for networking local area networks using protocols."

Wireless connection

"Any computer network that predominantly connects hardware via Wi-Fi, eliminating much of the need for physical cabling."

Wi-Fi

""Wireless connection to a network.

Requires a wireless access point or router.""

Bluetooth

"A method of exchanging data wirelessly over short distances - much shorter than Wi-Fi. Examples of typical Bluetooth use could be, headphones, car mobiles etc."

Encryption

"The process of altering a file so that the orignial data can only be read by the intended recipient.

Data is Enrypted into Cypher text and sent.

Data is then Decrypted upon receipt "

IP address

Internet Protocol Address: "A unique string of numbers separated by full stops. Identifies each computer using IP to communicate via a network."

MAC address

Media Access Control Address: "Used as a unique identifier for most network technologies including Ethernet and Wi-Fi."

Standards

"Standards allow hardware and software from different manufacturers to interact with each other."

Protocol

A set of rules that controls the transfer of data across a network

TCP/IP

""TCP provides error-free transmission between two routers.

IP routes packets across a wide area network.""

HTTP

""Protocol that controls the requesting and delivering HTML web pages.

Used when the information on a web page is not sensitive or personal.""

HTTPS

""Protocol that controls the requesting and delivering HTML web pages.

Used when the information on a web page is sensitive or personal.

The web page is encrypted""

FTP

File Transfer Protocol: "Used for sending files between computers, usually on a wide area network."

POP

"Used to download emails from a server.

Actually downloads the email from the server to a device. It is no longer available to downlaod to other devices. "

IMAP

A protocol used to access emails stored on a server. The email remains on the server so it can be accessed by multiple devices.

SMTP

Protocol used to send email to a mail server.